Affinity partitioning of membranes. Evidence for discrete membrane domains containing cholinergic receptor

Detecting extracellular carbonic anhydrase activity using membrane inlet mass spectrometry

Current research into the function of carbonic anhydrases (CAs) in cell physiology emphasizes the role of membrane-bound CAs such as CA IX, which has been identified in malignant tumors and is associated with extracellular acidification as a response to hypoxia. Here we present a mass spectrometric method to determine the extent to which total CA activity is due to extracellular CA in whole cell preparations. The method is based on the biphasic rate of depletion of (18)O from CO(2) measured by membrane inlet mass spectrometry. The slopes of the biphasic depletion are a sensitive measure of the presence of CA inside and outside of the cells. This property is demonstrated here using suspensions of human red cells in which external CA was added to the suspending solution. It is also applied to breast and prostate cancer cells, both of which express exofacial CA IX. Inhibition of external CA is achieved by the use of a membrane impermeant inhibitor that was synthesized for this purpose, p-aminomethylbenzenesulfonamide attached to a polyethylene glycol polymer.

Mammalian plasma membrane proteomics

Plasma membrane proteins serve essential functions for cells, interacting with both cellular and extracellular components, structures and signaling molecules. Additionally, plasma membrane proteins comprise more than two-thirds of the known protein targets for existing drugs. Consequently, defining membrane proteomes is crucial to understanding the role of plasma membranes in fundamental biological processes and for finding new targets for action in drug development. MS-based identification methods combined with chromatographic and traditional cell-biology techniques are powerful tools for proteomic mapping of proteins from organelles. However, the separation and identification of plasma membrane proteins remains a challenge for proteomic technology because of their hydrophobicity and microheterogeneity. Creative approaches to solve these problems and potential pitfalls will be discussed. Finally, a representative overview of the impressive achievements in this field will also be given.

Sterol carrier protein-2: new roles in regulating lipid rafts and signaling

Sterol carrier protein-2 (SCP-2) was independently discovered as a soluble protein that binds and transfers cholesterol as well as phospholipids (nonspecific lipid transfer protein, nsLTP) in vitro. Physiological functions of this protein are only now beginning to be resolved. The gene encoding SCP-2 also encodes sterol carrier protein-x (SCP-x) arising from an alternate transcription site. In vitro and in vivo SCP-x serves as a peroxisomal 3-ketoacyl-CoA thiolase in oxidation of branched-chain lipids (cholesterol to form bile acids; branched-chain fatty acid for detoxification). While peroxisomal SCP-2 facilitates branched-chain lipid oxidation, the role(s) of extraperoxisomal (up to 50% of total) are less clear. Studies using transfected fibroblasts overexpressing SCP-2 and hepatocytes from SCP-2/SCP-x gene-ablated mice reveal that SCP-2 selectively remodels the lipid composition, structure, and function of lipid rafts/caveolae. Studies of purified SCP-2 and in cells show that SCP-2 has high affinity for and selectively transfers many lipid species involved in intracellular signaling: fatty acids, fatty acyl CoAs, lysophosphatidic acid, phosphatidylinositols, and sphingolipids (sphingomyelin, ceramide, mono-di-and multi-hexosylceramides, gangliosides). SCP-2 selectively redistributes these signaling lipids between lipid rafts/caveolae and intracellular sites. These findings suggest SCP-2 serves not only in cholesterol and phospholipid transfer, but also in regulating multiple lipid signaling pathways in lipid raft/caveolae microdomains of the plasma membrane.

Aqueous polymer two-phase systems: effective tools for plasma membrane proteomics

Plasma membranes (PMs) are of particular importance for all living cells. They form a selectively permeable barrier to the environment. Many essential tasks of PMs are carried out by their proteinaceous components, including molecular transport, cell-cell interactions, and signal transduction. Due to the key role of these proteins for cellular function, they take center-stage in basic and applied research. A major problem towards in-depth identification and characterization of PM proteins by modern proteomic approaches is their low abundance and immense heterogeneity in different cells. Highly selective and efficient purification protocols are hence essential to any PM proteome analysis. An effective tool for preparative isolation of PMs is partitioning in aqueous polymer two-phase systems. In two-phase systems, membranes are separated according to differences in surface properties rather than size and density. Despite their rare application to the fractionation of animal tissues and cells, they represent an attractive alternative to conventional fractionation protocols. Here, we review the principles of partitioning using aqueous polymer two-phase systems and compare aqueous polymer two-phase systems with other methods currently used for the isolation of PMs.

Affinity partitioning for membrane purification exploiting the biotin-NeutrAvidin interaction. Model study of mixed liposomes and membranes

Biotinylated negatively charged liposomes as well as membranes were affinity partitioned in an aqueous poly(ethylene glycol)-dextran two-phase system using NeutrAvidin conjugated to dextran as affinity ligand. Both liposomes and membranes redistributed from top to bottom phase upon addition of NeutrAvidin-dextran. The presence of 35-60 mM Li2SO4 was necessary both to force the components into the top phase without ligand and for ligand-dependent redistribution into the bottom phase. Attaching biotin via a hexanamidohexanoyl spacer and an increased density of biotin or NeutrAvidin enhanced the affinity separation. The separation conditions in these model experiments provide a basis for affinity partitioning of membranes using other affinity ligands.

A convenient method for the synthesis of amine-terminated poly(ethylene oxide) and poly(epsilon-caprolactone)

A convenient synthetic route to prepare amine-terminated poly(ethylene oxide) (PEO) and poly(epsilon-caprolactone) (PCL) was described. The strategy involved two-step reactions, the condensation of hydroxyl-terminated PEO and PCL with N-benzyloxycarbonyl amino acid followed by the catalytic hydrogenation under mild conditions. NMR and GPC measurements indicated that the reactions proceeded nearly quantitatively. Amine-terminated PEO thus prepared was used to initiate the polymerization of alpha-(N(epsilon)-benzyloxycarbonyl-L-lysine) N-carboxy anhydride [lys(Z)-NCA], and the results confirmed that the reactivity of the amino group was high.

Effects of specific binding reactions on the partitioning behavior of biomaterials

Affinity partitioning is a special branch of biomaterials separations using aqueous two-phase systems. It combines the capability of diverse biomolecules to partition in aqueous two-phase systems using the principle of biorecognition. As a result, the macromolecule exhibiting affinity for a certain ligand is transferred to that phase where the ligand is present. This chapter describes the present status of the theoretical background of this approach and the properties of various natural and artificial compounds which act as affinity ligands in liquid-liquid systems. The affinity partitioning of proteins (enzymes and plasma proteins), cell membranes, cells, and nucleic acids are described as typical examples. The results are discussed in terms of theoretical understanding and practical application.

Partitioning and concentrating biomaterials in aqueous phase systems

Aqueous phase separation is a general phenomenon which occurs when structurally distinct water-soluble macromolecules are dissolved, above certain concentrations, in water. The number of aqueous phases obtained depends on the number of such distinct macromolecular species used. Aqueous two-phase systems, primarily those containing poly(ethylene glycol) and dextran, have been widely used for the separation of biomaterials (macromolecules, membranes, organelles, cells) by partitioning. The polymer and salt compositions and concentrations chosen greatly affect the physical properties of the phases. These, in turn, interact with the physical properties of biomaterials included in the phases and affect their partitioning. Specific extractions of biomaterials can be effected by including affinity ligands in the systems. The phase systems can also be used to obtain information on the surface properties of materials partitioned in them; to study interactions between biomaterials; and to concentrate such materials.

Effect of some poly(ethylene glycol)-bound and dextran-bound affinity ligands on the partition of synaptic membranes in aqueous two-phase systems

Ligands with an apparent affinity for various structural elements on the surface of synaptic membrane fragments have been bound to the polymers poly(ethylene glycol) and dextran. The ligand-polymer derivatives have been included in aqueous two-phase systems composed of water, poly(ethylene glycol) and dextran. The uneven distribution of the polymers resulted in the concentration of the polymer-bound ligand in one of the two phases. The effect of the ligand-polymer on the partition of membranes was studied by using synaptic membranes from calf brain, obtained by standard centrifugation methods. By using ligand-containing two-phase systems for nine-step counter-current distribution of membranes, it was shown that the distribution behaviour of various parts of the membrane preparation could be affected. The distribution was followed by determination of opiate binding, acetylcholinesterase, and total membrane (using protein and light-scattering measurements).

Affinity-mediated modification of electrical charge on a cell surface: a new approach to the affinity partitioning of biological particles

Polylysine has been covalently bound to human transferrin in a 1:1 molar ratio over a disulfide bond that can be easily split by reducing agents such as dithiothreitol. The association constant for the binding of the transferrin-polylysine derivative to transferrin receptors present on rat erythroblasts and the number of binding sites were identical to the corresponding values found for native transferrin. The incubation of the cells with transferrin-polylysine affected the partitioning of erythroblasts in a charge-sensitive aqueous two-phase system containing Dextran and polyethylene glycol. The polylysine part introduced a nonspecific influence on the partitioning that could be eliminated by preincubation of the cells with an excess of sialic acid. The partition ratio, G, of the erythroblasts changed with a factor of 1.9 for each set of 100,000 polylysine chains attached per cell.

Subfractions of membranes from calf brain synaptosomes obtained and studied by liquid-liquid partitioning

Synaptosomes isolated from calf brain cortex were lysed and fragmented by Yeda press treatment. The obtained membranes have previously been fractionated in a counter-current distribution process using a liquid-liquid two-phase system consisting of water, dextran, Ficoll and poly(ethylene glycol) [J. Chromatogr., 358 (1986) 147]. Using the fact that there are discrete membrane populations, a rapid preparative method for isolation of the two main fractions is presented in the present work, as well as a subfractionation of one of them using liquid-liquid extraction with dextran-bound Procion yellow HE-3G. The content of several membrane constituents, i.e. protein, acetylcholinesterase, succinate dehydrogenase and ATPase, as well as opiate binding, were determined for the three fractions. Counter-current distribution of the fractions elucidates their heterogeneity and the effectiveness of the purification.

Heterogeneity of synaptosomal membrane preparations from different regions of calf brain studied by partitioning and counter-current distribution

1. Membranes obtained by lysis and Yeda-press treatment of synaptosomes (nerve endings) from cortex, caudateus nucleus, and hippocampal region of calf brain have been studied by partitioning within a liquid-liquid aqueous two-phase system consisting of water, dextran, Ficoll, and poly(ethylene glycol). 2. The partitioning of membranes was sensitive to the presence of a dextran-bound dye, Procion yellow HE-3G, in the lower phase. 3. The two-phase system was used for counter-current distribution to study the heterogeneity of the synaptic membranes from the three regions of the brain and to separate the membranes into fractions. 4. The obtained counter-current distribution profiles strongly depended on the region of the brain from which the membranes were isolated. 5. The membrane fractions obtained showed marked differences in their SDS electrophoresis pattern.

Partition of synaptic membranes in aqueous two-phase systems at subzero temperatures by using anti-freeze solvent

The freezing point of aqueous two-phase (liquid-liquid) systems containing water, dextran and poly(ethylene glycol) has been lowered by including glycerol. Biological membranes, obtained by fragmentation of a crude synaptosomal preparation from calf brain cortex, have been included in the two-phase systems. The effects of temperature and the concentration of glycerol on the partition of the membranes within the systems have been investigated. Considerable stabilisation of the membranes was noticed when they were partitioned at -10 degrees C compared with 0 degrees C. The influences of glycerol, ethylene glycol, N,N-dimethylformamide and tetrahydrofuran on the phase-forming properties of the systems and on enzyme activities are also presented. Possible use of the above systems for studies and separation of biological membranes are discussed.

The intactness and orientation of acetylcholine receptor-rich membrane from Torpedo californica electric tissue

By a mild and highly reproducible fractionation of Torpedo californica electric tissue, we prepared membrane which was 30 times enriched in nicotinic acetylcholine receptor (AChR). This preparation was neither alkali-stripped nor reconstituted and consequently contained nu (43-kDa protein), which is associated with the cytoplasmic aspect of the receptor. We tested this membrane for the presence of sealed vesicles and determined the orientation of these vesicles by combining three methods. Two of these methods were based on the accessibilities, in the presence and absence of detergent, of the extracellular acetylcholine binding site to alpha-bungarotoxin and of the intracellular nu to trypsin. These two methods are specific for AChR-containing membrane. The third method was morphometry of electron micrographs, by which we estimated the proportion of sequestered membrane. These methods taken together indicated that approximately 45% of the AChR-containing membrane was in the form of leaky vesicles or sheets, 33% was sealed right-side-out vesicles, 11% was sealed inside-out vesicles, and 11% was sequestered within multilamellar or multivesicular vesicles. The complexity of this membrane needs to be taken into account in sidedness studies of the AChR.

Analysis of the domain structure of membranes by fragmentation and separation in aqueous polymer two-phase systems

This paper consists of three parts. The first describes theoretically a general strategy for fragmentation and separation of membranes which can be used in the elucidation of their structure and function. The second part describes a practical separation method, partition in liquid aqueous polymer two-phase systems, which can be used for separation of macromolecules and membrane particles of biological origin. The third part gives examples of the application of this method to membrane vesicles, and how this separation in combination with the strategy described in the first part can be used for analysis of the structure of biological membranes.

Alkylation of free sulfhydryls fortifies electroplax subsynaptic structures

The cysteine-rich 43,000-dalton peripheral membrane protein, nu 1, is localized at the cytoplasmic face of electroplax and muscle cholinergic synapses, where it is thought to play an important role in the endplate supramolecular structure. The peripheral membrane protein properties of nu 1 are inferred by its removal from nicotinic cholinergic membranes by the action of mild alkali or lithium diiodosalicylate. An interesting property of nu 1 is its high concentration of free sulfhydryl groups, whose exact role in synaptic structure is still largely unknown. Alkylation of free sulfhydryls with N-ethylmaleimide (3 mM) has a profound effect on the association of nu 1 with synaptic membranes, rendering nu 1 unextractable by pH 11 treatment or by lithium diiodosalicylate and, concomitantly, decreasing nu 1's electrophoretic mobility on sodium dodecyl sulfate-polyacrylamide gels. Iodoacetamide and iodoacetate have similar effects, but at concentrations 10- to 100-fold higher than required for N-ethylmaleimide. Furthermore, sulfhydryl modification also stabilizes the association of nicotinic receptor subunits with the detergent-insoluble cytoskeleton. N-Ethylmaleimide treatment increases the fraction of insoluble receptor molecules on extraction with Triton X-100, sodium cholate, or octylglucoside. These results suggest an important role of sulfhydryl groups in the structural stability of the postsynaptic cholinergic membrane.

Effect of dextran- and poly(ethylene glycol)-bound procion yellow HE-3G on the partition of membranes from calf brain synaptosomes within an aqueous two-phase system

Membranes obtained by lysis and Yeda-press treatment of synaptosomes (nerve endings) from calf brain cortex have been partitioned within the aqueous phases (and the interface between them) of a Ficoll-dextran-poly(ethylene glycol)-water two-phase system. By introducing the dye Procion yellow HE-3G in the upper phase, bound to poly(ethylene glycol), or in the lower phase, bound to dextran, the partition of the membranes could be strongly affected. The influence on the partition was more pronounced when the dye was bound to dextran. By using a number of two-phase systems in a counter-current distribution process, it was shown that the membrane preparation was inhomogeneous and that the fractions obtained differed in their contents of acetylcholinesterase, succinate dehydrogenase and ATPase. The affinity partitioning effect depended strongly on the concentration of polymer-bound dye. An optimum dye concentration was found when Procion yellow HE-3G was bound to poly(ethylene glycol). When the same dye was bound to dextran, the number of dye molecules per dextran molecule influenced the effectiveness of the extraction.

Creatine kinase isoenzymes in Torpedo californica: absence of the major brain isoenzyme from nicotinic acetylcholine receptor membranes

Creatine kinase, actin, and nu 1 are three proteins of Mr 43 000 associated with membranes from electric organ highly enriched in nicotinic acetylcholine receptor. High levels of creatine kinase are required to maintain adequate ATP levels, while actin may play a role in maintaining the synaptic cytoskeleton. Previous investigations have prompted the conclusion that postsynaptic specializations at the receptor-enriched membrane domains in electroplax contain the brain form of creatine kinase rather than the form of creatine kinase predominantly found in muscle. We have examined this conclusion by purifying Torpedo brain creatine kinase to virtual homogeneity in order to examine its immunochemical, molecular, and electrophoretic properties. On the basis of immunological cross-reactivity and isozyme analysis, the receptor-associated creatine kinase is identified to be of the muscle type. When the molecular characteristics of Torpedo brain and muscle creatine kinase are compared, the brain enzyme is positioned at a more basic pH during chromatofocusing and on two-dimensional gel electrophoresis (pI = 7.5-7.9). Furthermore, electrophoretic mobilities of the brain and muscle forms of creatine kinase differ in sodium dodecyl sulfate electrophoresis: the brain isozyme of creatine kinase has lower apparent molecular weight (Mr 41 000) when compared with the muscle enzyme (Mr 43 000). On the basis of the results of our current investigations, the hypothesis that the brain isozyme of creatine kinase is a component of the postsynaptic specializations of the Torpedo californica electroplax must be abandoned. Recent sequence data have established close homology between Torpedo and mammalian muscle creatine kinases. On the basis of electrophoretic criteria, our results indicate that a lower degree of homology exists between the brain isozymes.

Palytoxin-induced permeability changes in excitable membranes

Palytoxin, a toxin isolated from the Caribean corrall Palythoa caribaeorum, increases the cation permeability of excitable membranes in vitro. Three membrane systems have been investigated: axonal membranes from crayfish walking leg nerves, membranes rich in nicotinic acetylcholine receptor isolated from Torpedo californica electric tissue and, for control, artificial liposomes. Ion permeability of the latter was not affected by palytoxin, but with both biological membranes an increase in cation permeability was observed at a palytoxin concentration of 0.14 microM. Palytoxin-induced cation flow through the axonal membrane was not inhibited by tetrodotoxin, indicating that the voltage-dependent sodium channels were not involved. The effect of palytoxin on the receptor-rich membranes was not blocked by alpha-bungarotoxin, a competitive antagonist of the nicotinic acetylcholine receptor, nor by triphenylmethylphosphonium, a blocker of the receptor-ion channel. But with both the axonal and the receptor-rich membranes ouabain was an inhibitor of the palytoxin-induced cation flow. Evidence is presented that it is not the (Na+ + K+)-ATPase which is affected by palytoxin as has been postulated for similar observations with non-neuronal membranes (Chhatwal, G.S., Hessler, H.-J. and Habermann, E. (1983) Naunyn-Schmiedeberg's Arch. Pharmacol. 323, 261-268).

Affinity partitioning and centrifugal counter-current distribution of membrane-bound opiate receptors using naloxone-poly(ethylene glycol)

Crude synaptic membranes isolated from calf brain cortex were subjected to an aqueous two-phase system and the partition of the various membrane constituents and activities between the phases were studied. These constituents were phosphate, cholesterol and protein. The activities measured were acetyl-cholinesterase, succinate dehydrogenase, 2',3'-cyclicnucleotide-3'-phosphohydrolase and stereospecific opiate-binding. The successful fractionation of the membranes was achieved by the use of an aqueous two-phase system in a counter-current distribution process. A ligand bound to poly(ethylene glycol) with an affinity for opiate receptors was synthesized by reacting 6-aminonaloxone with tresylpoly(ethylene glycol). The ligand-polymer was used to extract membrane-bound opiate receptors into the upper, poly(ethylene glycol)-rich phase. This use of affinity partitioning resulted in membrane fractions with a 3-4 fold higher ability to bind stereospecifically etorphine than the original preparations of synaptic membranes.

Liquid-liquid extraction of membranes from calf brain using conventional and centrifugal counter-current distribution techniques

Neural membranes isolated from calf brain have been partitioned in aqueous two-phase systems containing dextran and polyethyleneglycol. When the partition was repeated several times, using counter-current distribution technique, the distribution of the membranes between the upper phase and the interface changed in a non-ideal manner and in favour of the interface. By using a centrifugal counter-current distribution device the time for the experiment could be reduced by a factor of 7-8 and the distribution was similar to what could be expected for ideally behaving membranes. The time-dependent change of the membranes is discussed in terms of aggregation and lateral membrane perturbations. Despite this effect a certain fractionation has been achieved as deduced from analysis of cholesterol content, opiate receptor activity and acetylcholinesterase activity along the counter-current distribution row of fractions. Compared to the starting material these activities were enriched some two-fold in certain fractions.

Affinity partitioning of phosphofructokinase from baker's yeast using polymer-bound Cibacron blue F3G-A

1. Phosphofructokinase from baker's yeast is partitioned between the phases of an aqueous two-phase system, containing dextran (Mr = 500000) and poly(ethyleneglycol) (Mr = 6000), in favour of the dextran-rich phase. By covalent binding of the dye Cibacron blue F3G-A to poly(ethyleneglycol) the enzyme can be extracted to the phase rich in this polymer, i.e. affinity partitioning. 2. The affinity partitioning effect, measured as the logarithmic increase of the partition coefficient by introducing polymer-bound Cibacron blue, depends on several factors. The influence of dye-polymer concentration, polymer concentration, polymer molecular weight, kind of salt and salt concentration, pH and temperature has been studied. 3. The effect of ATP, ADP, AMP, ITP, fructose 1,6-bis-phosphate and fructose 6-phosphate show large differences in the binding strength of these substances to the Cibacron blue binding sites. AMP cannot compete with Cibacron blue while ATP is strongly competing. 4. The use of affinity partitioning for enzyme isolation and determination of ligand binding is discussed, as well as possible mechanisms concerning this type of liquid/liquid extraction.

Extraction of peripheral proteins from nicotinic acetylcholine receptor-enriched membranes

The solubilisation of membrane proteins from nicotinic acetylcholine receptor-enriched membranes from the electric organ of Torpedo marmorata was studied. Chaotropic ions were shown to be ineffective in extracting peripheral proteins from these membranes. Two different anhydrides, 2, 3-dimethylmaleic and 3,4,5,6-tetrahydrophthalic anhydride, released certain peripheral membrane proteins but not the integral receptor protein. Treatment of membranes containing greater than 3 nmol alpha-bungarotoxin binding sites per mg protein with anhydride resulted in a 43 kDa polypeptide as the major constituent of the solubilised material. The nature of the 43 kDa polypeptide is discussed. Gentle anhydride treatment did not change the alpha-bungarotoxin and carbamoylcholine binding properties of the receptor.

LM fibroblast plasma membrane subfractionation by affinity chromatography on con A-sepharose

Affinity chromatography was used to determine the heterogeneity and orientation of plasma membrane vesicles isolated from LM fibroblasts subjected to Dounce homogenization. Two plasma membrane subfractions were obtained by Con A-Sepharose affinity chromatography of LM fibroblast plasma membranes prepared by Dounce homogenization. The desmosterol-phospholipid molar ratio, the phospholipid composition, and the phospholipid fatty acid composition were almost identical between the two fractions. However, the lipid to protein ratio was almost 2-fold greater in the nonadherent fraction A. The binding of fluorescein-concanavalin A was the same in both fractions indicating a right-sided-out orientation of the vesicles. Similarly and asymmetric distribution of phosphatidylethanolamine in both membrane fractions was the same. In contrast, sialic acid content, 5'-nucleotidase activity, and (Na+ + K+)-ATPase activity were 47%, 3.7-fold, and 2.5-fold greater, respectively, in the nonadherent, lipid-rich fraction A. Structural properties of the two membrane fractions determined by fluorescence polarization and arrhenius plots of trans-parinaric acid fluorescence were similar. These results indicate that concanavalin-A affinity chromatography separates two membrane fractions differing in sialic acid content, lipid content, and enzyme profile but having the same right-side-out orientation.